Young sunflower plants reorient at night in anticipation of dawn.
By Rakesh Agrawal, ‘Ridh
Who is not mesmerized by blooming, smiling and invigorating sunflowers and who has stopped wondering why they always face the sun and turn their faces to face the sun—well, only until they’re infants as after reaching maturity, they stop tracking the sun and only face east?
This behavior of these ethereal flowers remained a mystery until now when the scientists have answered this question, central to their charm.
Researchers in Science magazine say the young plant’s sun-tracking or heliotropism is actually circadian rhythms or the behavioral changes tied to an internal clock that humans also have, which follow a roughly 24-hour cycle. A young flower faces east at dawn and greets the sun, then slowly turns west as the sun moves across the sky. Again, in the night, it slowly turns back to the east and starts the cycle again. (See: http://www.npr.org/sections/thetwo-way/2016/08/05/488891151/the-mystery-of-why-sunflowers-turn-to-follow-the-sun-solved)
Body clocks also ticks in this flower and, “It’s the first example of a plant’s clock modulating growth in a natural environment,” UC Davis professor and study co-author Stacey Harmer said. This turning of the flower is actually a result of different sides of the stem elongating at different times of day.
“Growth rates on the east side were high during the day and very low at night, whereas growth rates on the west side were low during the day and higher at night,” the journal notes, hence, “The higher growth rate on the east versus west side of the stem during the day enables the shoot apex to move gradually from east to the west. At night, the higher growth rate on the west side culminates in the apex facing east at dawn.”
A young sunflower plant not only tracks the sun during the day but also reorients at night in anticipation of dawn.
Mature sunflowers respond differently to the sun as it has achieved its full growth and the researchers compared mature flowers facing east with those they turned to face west, and found that the east-facing blooms attracted five times as many helpful pollinators.
“Just like people, plants rely on the daily rhythms of day and night to function,” says Anne Sylvester, director of the National Science Foundation’s Plant Genome Research Program.
Hence, young sunflowers actually dance as each day, when they trace the path of the sun across the sky, turning their faces 180 degrees from east to west.
And their slow, graceful movements continue at night. After the sun sets they reorient themselves, slowly twisting their heads back to the east in anticipation of dawn.
(See: http://www.latimes.com/science/sciencenow/la-sci-sn-sunflowers-direction-20160804-snap-story.html).